1. Field of the Invention
This invention relates to a digital frequency generator or oscillator and in particular to a digital technique for shifting the frequency of the digital oscillator in accordance with the binary data to be transmitted.
2. Description of the Prior Art
Frequency shift keyed oscillators have been used for the transmission of binary information for many years. One complication is concerned with the communication of digital information in certain systems which require the conversion of the digital information to the audio-frequency range. As a result, numerous techniques have been developed for achieving this end. Generally, these techniques are concerned with converting two-level binary data into audio tones corresponding to the respective binary levels. Some of these techniques include switching between multiple oscillators; switched phase shift RC oscillators; voltage-controlled multivibrator oscillators; and variable reactance, phase shift RC oscillators among others. Because of stability requirements, the common approach was to use very stable temperature compensated LC oscillators. In this case, frequency shifting was achieved by switching in and out a second capacitor.
RC oscillators are not new, per se, but the circuits generally available do not have sufficient stability for applications with low bit rates in the voice-frequency band.
It is an object of the present invention to provide an improved frequency shift keyed oscillator which has very good frequency stability and which can be frequency shifted by an incoming binary data signal without producing undesirable transients.